An integrated circuit (IC) die is typically packaged between a substrate and a lid. The substrate provides electrical connections between each of the contact pads of the IC die and external connections of the package. Furthermore, many packages are also required to efficiently conduct heat from the IC die to the outside of the package, primarily through the package lid, which in many cases also acts as a heat spreader.
Current state of the art in package design of lidded packages include in-plane single material lids with or without a cavity. Sometimes, cavity shaped lids are constructed out of two or more materials, however even in these cases, the horizontal plate portion is constructed out of a single material. Single material construction provides mechanical integrity, manufacturing simplicity, and low cost. However, as a result, there exists a thermo-mechanical problem when attaching a single material lid to a package with a large silicon die that generates significant heat.
FIG. 1 shows an exemplary existing package assembly 100. Assembly 100 includes package 110 mounted to circuit board 104 via socket 102. Package 110, which is shown in more detail in FIG. 2, includes substrate 112, IC die 114, and lid 116. Socket 102 is optional as some assemblies have package 110 soldered directly to circuit board 104. Package 110, socket 102, and circuit board 104 are sandwiched between heat sink 108 and bolster plate 109. Heat sink 108 receives heat from package 110 and dissipates the heat into the ambient. Bolts 120 apply a force via springs 122 to package 110 thereby ensuring good electrical contact between substrate 112 and circuit board 104 via socket 102. Other mechanical connections are known as well.
An electrically powered fan (not shown) may also be provided to provide enhanced airflow across heat sink 108 to further increase heat dissipation. In some systems, in place of heat sink 108 may be a heat exchanger having circulating fluid, a heat pipe, or other means for removing heat from package 110. The term, “heat sink” as used herein will refer to any such heat removal device.
Heat passes from IC die 114 to lid 116 via a thermally conductive adhesive 124, which is bonded to both IC die 114 and lid 116 to provide a continuous heat path. Heat then passes from lid 116 to heat sink 108 by way of a thermal transfer compound 126 or other thermal medium. Lid 116 is attached to substrate 112 by adhesive 118 or other connecting means.
Lid 116 is required to have good thermal conductivity, and is also required to be sufficiently strong to withstand the forces applied by bolts 120 and springs 122. Depending on the requirements, the pressure exerted on lid 116 could be as high as 300 pounds per square inch and higher. Such pressures are necessary, e.g., to ensure good electrical connection between contact lands 121 on substrate 112 and corresponding ones (not shown) on socket 102. To accommodate strength and thermal conductivity requirements, prior art lids were therefore typically made of metals or ceramic materials (AlSiC) with good thermal conductivity and strength properties.
However, a problem with materials having good thermal conductivity and strength has been an inevitable mismatch in the coefficient of thermal expansion (CTE) between lid 116 and other components. This is because thermally advanced materials having high thermal conductivity have, as a byproduct of their construction, a low CTE, and hence, a resulting thermo-mechanical mismatch with high CTE substrate and low CTE silicon die, thus creating a contradiction between thermal and reliability requirements.
For example, the silicon IC die may have a coefficient of thermal expansion of 2.5 (10−6)/° C. whereas the lid may have a coefficient of, for example, from 6 (10−6)/° C. to 17 (10−6)/° C. This difference results in relative movement between lid 116 and IC die 114, which can result in delamination of thermally conductive adhesive 124. This delamination represents a thermal break, which can significantly reduce the conductivity of the thermal path within package 110. The risk of delamination is increased with newer IC dies such as multi-core processors with large caches, which have a large surface area and greater power dissipation.
A need therefore exists to provide an IC die package that reliably provides an unimpeded thermal path for heat to flow from the IC die to the exterior of the package.